1. Field of the Invention
The present invention relates to a novel zinc oxide thin film, a transparent conductive film and a display device, using the same. Specifically, the present invention relates to a thin film device that is thin, lightweight and high precision and that uses a highly efficient, long-life transparent conductive film. More specifically, the present invention relates to a liquid crystal display, a plasma display panel, a field emission display, an organic electroluminescent display, and the like.
2. Description of the Related Art
Recently, demand for lightweight, highly colorful, highly bright, inexpensive and small-sized flat panel display (FPD) is increasing along with the spread of various cell phones, mobile terminals, mobile computers, car navigation systems, and the like. In addition, also at home and in offices, flat panel displays such as space-saving desktop displays and wall hanging televisions are now being substituted for conventional CRT displays. In particular, due to the diffusion of the high speed Internet and the progress of digital broadcasting, digital signal transmission of roughly several hundreds to several gigabits/second has been commercialized in both wire and wireless transmissions. Thus, an era is coming when general users exchange an extremely large set of information in real time. As such, these flat panel displays demand digital signal processable and high-speed properties in addition to lightweight, high colorfulness, high brightness, and low cost more than conventional displays do. These FPDs that are known include liquid crystal displays (LCDs), plasma display panels (PDPs), field emission displays, organic light emitting diode (OLEDs) displays, and the like. In these displays, application of an electric field and an electric current to each display pixel causes the generation of light, and adjusts the light amount. At the same time, such displays use a transparent electrode which transmits visible light, and through which an electric current can flow because the displays need to take light out of the device.
Transparent electrode materials are roughly classified into crystalline and amorphous materials. The crystalline materials that are known are indium oxide (In2O3) based materials including indium-tin oxide (ITO), zinc oxide (ZnO) based materials, tin oxide (SnO2) based materials, titanium oxide (TiO2) based materials and cadmium-tin oxide (CdSnO4) based materials. In addition, the amorphous materials include indium zinc oxides (IZO: amorphous phase appearing as an intermediate composition of In2O3 and ZnO. Typically, 80% of In2O3). Of these, overwhelmingly spreading transparent electrode material is ITO (a substance in which tin Sn is substituted by 10 to 20% for In in In2O3 crystal), which is a kind of indium oxide (In2O3) based materials. A typical technique of forming ITO to a thin film as a transparent electrode is a sputtering method. This technique involves placing a target of ITO serving as a transparent electrode material in a vacuum chamber such that the target opposes a substrate on which a thin film is to be formed. Then, the target material is vaporized by use of argon (Ar) or oxygen (O2) gas as a sputtering gas, and thereby the material is deposited on the substrate. This technique integrally forms an ITO thin film with a film thickness of several tens to several hundreds nm on a large substrate of a several m-square size. In addition, a transparent electrode formed of ITO is stable, that is, its characteristics such as properties matching with other members making up FPD and patterning processability by a photoresist are not changed even by heat treatment and the like during the fabrication processes.
However, ITO contains indium (In), which is a rare metal, as a main component. The reserve of In itself is limited, and In is only produced as a by-product of a zinc ore. Thus, it is feared that the resources will be depleted. Hence, an alternative transparent electrode material instead of ITO is craved. Of transparent electrode materials in addition to ITO, SnO2 based materials and TiO2 based materials have the electrical conductivity larger than that of ITO by more than one order of magnitude, and are poor in processability. Due to toxicity in Cd, CdSnO4 based materials are hardly studied. Because IZO based materials have In as a main component, they are not said to be alternative materials not using In. Substances that can exhibit transparency and electrical conductivity equivalent to those of ITO are ZnO based materials, and are the most powerful candidates of the alternative material.
General physical properties of zinc oxide (ZnO) crystals are summarized in, for example, “Studies on Zinc oxide,” Chapter 2, General description of ZnO, Research Report Vol. 50, Science and Technology Agency, Inorganic Material Laboratory, 1987. For example, zinc oxide (ZnO) has a wurtzite form (hexagonal crystal) structure that is produced as zincite in nature, and its bond pattern is an intermediate between ionic bond and covalent bond. The a-axis=3.249 Å, the c-axis=5.207 Å, and the polarity lies in the c-axis direction. When zinc oxide is cleaved perpendicularly to the c-axis direction, the (0001) plane on which Zn is exposed to the surface (called the Zn plane), and the (0001) plane on which O is exposed to the surface (called the O plane) are obtained. The others include the nonpolar (1120) plane and the (1010) plane. A terrace and a step are present on the surface in many cases. Moreover, the followings are known. Under a high pressure exceeding 100 kbar, NaCl form (cubic crystal) structure appears. Although ZnO produced by oxidation of Zn deposited on the copper line has the NaCl form structure as under this high pressure, it shows a crystal structure having a 10% larger lattice constant. ZnO is the group-IIb-VIb wide-gap n-type semiconductor belonging to the hexagonal crystal of the wurtzite form in a single crystal state, and its band gap is 3.2 eV, and thus ZnO is transparent in the visible region. The conduction band is formed by the σ anti-bonds of Zn 4s and O 2p. For impartment of electrical conductivity, a free carrier needs to be imparted to this conduction band. There are two kinds of free carriers of the electronic conductive ZnO: one is of a case where oxygen vacancies in the ZnO crystal or excessive Zn atoms between lattices are formed, and thus the proportion of Zn in the ZnO becomes larger than 0; and the other is of a case where Zn or O is substituted by an element the valance number of which is large (group-IIIb=B, Al, Ga, In, group-VIIb=F), and residual bond electrons becomes free electrons. Typical substitution elements are Al and Ga. The substitution ratio is at most about 5%. Both the effects of being high in the Zn proportion and addition of a substitution element are required in order to obtain electrical conductivity equivalent to that of ITO.
It has been long recognized that a zinc oxide crystal of a wurtzite form has a structure produced by alternately laminating a zinc atomic layer only formed of zinc and an oxygen atomic layer only formed of oxygen, in its c-axis direction. Depending on whether the uppermost surface layer is a zinc layer or an oxygen layer, the uppermost surface layer is called a zinc surface or an oxygen surface, respectively, and its electric polarity is positive for the zinc surface and negative for the oxygen surface. Thus, the surfaces have greatly different physical properties from each other, and both the surfaces of the zinc oxide single crystal have been studied.
In addition, difference in the polarity plane of the ZnO single crystal also appears in adsorption of and chemical reactivity with exterior substances. The techniques of making this ZnO a thin film that are adopted include, in addition to a sputtering method as for ITO, a variety of thin film fabricating methods such as reactive plasma deposition (RPD), chemical vapor deposition (CVD), molecular beam deposition (MBD) and pulsed laser deposition (PLD). For example, Japanese Patent Application Publication No. Hei 7-106615 describes a ZnO film fabricating method by molecular beam deposition. This technique fabricates a film while doping Zn or O with an element with a large valence number (group-IIIb=B, Al, Ga, In, group-VIIb=F, Cl, Br, I); however, the film fabricating speed is slow in molecular beam deposition, and a vacuum chamber also needs a super vacuum state. For these reasons, the technique is not suitable as a film-fabricating method for FPD. Generally, ZnO is high in crystallinity, and thus is crystallized from the vicinity of the substrate interface, the substrate being an amorphous substrate made of glass, metal, organic resin, or the like. A large number of crystal nuclei are generated, and the orientations are at random, whereby ZnO becomes a microcrystal at an initial stage of thin film growth. Therefore, it is thought that the ZnO needs to grow till its film thickness becomes 100 nm or larger for the ZnO becoming a denser crystalline film. A general technique in order for such a thin film to grow to a highly crystalline state from a substrate interface, is to restrain the number of crystal nuclei at an initial stage of growth, and then to cause the nucleic to grow as calmly as possible. However, it is desired that, in considering the metric size of today's substrate for FPDs and the like, such transparent electrodes be formed into a thin film as high-speedily as possible for mass production improvement. As for ITO transparent electrodes, films are formed by a sputtering method, and ITO should have the properties as a transparent electrode, the properties being the same as those of a thinner film, even if the film thickness of the ITO exceeds 100 nm from the substrate interface due to its high crystallinity. As such, a transparent electrode material in place of ITO needs to be a high crystalline film while growing from the vicinity of the substrate interface to be a predetermined thick film, even if a thin film fabricating technique with high mass productivity is used.